An official picture showing the Queen at work at her desk has been released to mark the moment she becomes the longest reigning monarch in British history.

Mary McCartney, the daughter of former Beatle Sir Paul McCartney, has photographed the Queen sitting at her desk in Buckingham Palace working on matters of state to commemorate the milestone.

There had been speculation a celebratory private event was planned but it is understood that in keeping with the Queen's business-as-usual approach, today will be a normal working day for the monarch with no special party.

At this time of year the Queen is taking her traditional summer break at her private Scottish home at Balmoral Castle.

Today she will open the new Scottish Borders Railway and take a steam train ride on the new £294m railway with Scotland's First Minister Nicola Sturgeon.